Resistive attenuator



y 6, 1967 E. E. WEBSTER 3,320,571

RES ISTIVE ATTENUATOR Filed Sept. 16, 1963 Ill/P07 Ill/P117 ,eg 01127117 wwmv INVENTOR. [pl/04w 51169575 United States Patent 3,320,571 RESISTIVE ATTENUATOR Edmund Ernest Webster, Barnfield, Marlborough, England, assignor to Plessey-UK Limited, Essex, England, a British company Filed Sept. 16, 1963, Ser. No. 309,163 Claims priority, application Great Britain, Sept. 21, 1962, 35,979/ 62 Claims. (Cl. 338-89) This invention relates to resistive attenuators and has for one of its principal objects the provision of a continuously variable resistive attenuator of relatively simple and improved construction.

In accordance with this invention, a resistive attenuator is provided which includes a body portion of resistive material with first electrical terminal means being connected to the body of resistive material to provide a common input and output terminal and second input terminal means being connected to the resistive body portion and electrically insulated from the first terminal means so that the electrical path between the first and second terminal means is through the resistive body portion. Second output terminal means is arranged for engagement with the resistive body portion and for movement thereon for adjustment of the attenuator. In a preferred construction of this invention the common input and output terminal can be provided by a layer of conductive material on one of the major surfaces of the resistive body portion with the second output terminal engaging the opposite major surface and the second input terminal engaging an edge of the resistive body. Preferably, the opposite major surface of the resistive body, which is engaged by the second output terminal, is coated with a relatively thin layer of resistive material and characterized by having a substantially lower specific resistivity than the material of the resistive body portion. The second input layer is arranged to engage the relatively low specific resisitivity layer along one of its edges.

The novel features of this invention are set forth in the appended claims. The invention itself, together with additional objects and advantages thereof, will be more clearly understood from a reading of the following description in connection with the accompanying drawing wherein preferred embodiments of the invention are illustrated and in which:

FIG. 1 is a generally schematic, longitudinal section through an embodiment of this invention;

FIG. 2 is a circuit diagram of an artificial line composed of resistive elements;

FIG. 3 is an axial section through an alternative form of resistive attenuator constructed in accordance with this invention;

FIG. 4 is a section view along line 4-4 of FIG. 3; and

FIG. 5 is a section view along line 5-5 of FIG. 4.

With particular reference to the drawing, a continuously variable resistive attenuator constructed in accordance with this invention is illustrated as including a body 10, generally in the form of a rectilinear bar. Preferably, body comprises a composition resistor including a resistive body portion 12 having oppositely facing surfaces 14 and 16 and relatively spaced end edges 18 and 20. Resistive body 12 can be formed of a suitable resistive material, such as a carbon or metal-loaded plastic, and is coated along surface 16 by a layer 22 of suitable electrically conductive material such as a silver-loaded plastic. A layer 24 is suitably coated on surface 14 of the main body portion.

Layer 24 also consists of a suitable resistive material, such as carbon or metal-loaded plastic, having a specific resistivity lower than that of body portion 12 and is relatively thin in comparison to resistive body portion 12. As an example of relative thickness of the resistive body portion 12 and layer 24, a ratio of approximately 2:1 has given satisfactory results and, in this regard, layer 22 can be the same thickness as layer 24 or have the same ratio to resistive body portion 12 as layer 24, if desired.

Bar 16 is secured in a base portion 26 of suitable electrical insulating material. It will be noted that a terminal member 28 is connected to layer 22 and extends through insulating body 26 and, as will appear more clearly hereinafter, terminal 28 and layer 22 cooperate to provide a common input and output terminal for the resistive attenuator.

An additional layer 30, of electrical conducting material similar to layer 22, is arranged to engage one of the end edges of layer 24. Preferably layer 30 engages only the end edge of layer 24. A terminal member 32 extends from layer 30 through insulating base 26 to complete a second input terminal for the attenuator. With this arrangement layer 30 and terminal member 32 provide terminal means in electrical engagement with layer 24 and insulated from layer 22 so that the circuit between the input terminals is established through resistive layer 24 and resistive body portion 12. A slider contact, illustrated schematically as arrow 34 in FIG. 1, provides the second output terminal. The slider contact is slidable along the outer surface of layer 24 toward and away from layer 30 to provide for continuous adjustment of the resistive attenuator.

With reference to FIG. 2, assuming an input to be connected between terminals 28 and 32 and an output to be connected between terminal members 28 and contact 34, the resistive attenuator of FIG. 1 can be considered as the electrical equivalent of the illustrated artificial line circuit. More particularly, the circuit comprises a conductor 36 including a plurality of series connected resistive elements R1 and connected to a second, non-resistive conductor 38 at the junction between adjacent resistances R1 by bridge elements R2. To better relate the circuit of FIG. 2 with the structure of FIG. 1, resistive conductor 36 corresponds to layer 24 having a value in the circuit of ARI, the bridge elements R2 correspond to main resistive body portion 12 having a value in the circuit of 1/AR2. The end of conductor 36 corresponds to the input terminal made up of layer 30 and terminal member 32, conductor 38 corresponds to layer 22 and terminal member 28 to provide the common input and output terminal, and sliding contact 34 is engageable with conductor 36 to provide an adjustable output terminal. Thus, the circuit of FIG. 2 constitutes the exact electrical equivalent of the structure of FIG. 1 if the number of elements R1 and R2 is increased infinitely and the magnitude of each of the elements proportionally increased.

It will be observed that apart from the composition of the various layers, the construction of the continuously variable attenuator illustrated in FIG. 1 is very similar to that of a potentiometer having a track of resistive composition. In fact this invention may be structurally modified in accordance with various known constructional forms of such potentiometer, and one practical form of the invention based on a rotary-potentiometer construction is illustrated in FIGS. 3-5.

In FIG. 3, a base 40 of electrical insulating material supports a semi-circular composite resistor track 42 comprising a resistive body 44 and a conductive bottom layer 46 coated on and extending coextensively with body 44. A resistive layer 48 is provided on the opposite surface of body 44 and a terminal portion 50 engages one of the opposed end edges of composite track 42. Terminal 50 comprises a layer 52 of electrically conductive material engaged along the end edge of layer 48 and electrically insulated from layer 46, see FIG. 5. The construction of the composite resistor body of the embodiment of FIGS. 35 is practically identical to that of FIG. 1, in fact FIG. 1 could be considered as a developed sectional view through the resistor track 42.

In this embodiment, a terminal pin 56 is electrically connected to conductive layer 46 and extends through insulating base 40 for electrical connection with metal base 58. A second terminal pin 60 extends from layer 52 through supporting base 40 to form the center conductor of a coaxial connector 62 the outer conductor of which is formed by a bushing portion 64 connected to metal base 58. A brush member 66 is supported in engagement with terminal portion 50 and for movement on layer 48 and corresponds to the slidable wiper contact 34 of the embodiment illustrated in FIG. 1. More particularly, brush 66 is supported in a resilient brush arm 68 which is in turn connected to the inner end of a control knob 70 which extends through and is rotatable in an aperture 72 in an outer housing 73 of the attenuator. The housing 73 is suitably attached to support base 40 to complete the exterior construction of the attenuator. A second brush member 74 is also supported from brush arm 68 and is arranged for continuous engagement with a contact plate 76 secured in base 40. A terminal pin 78 extends from contact plate 76 to a coaxial connector 80 to form its center contact with the outer contact thereof being provided by bushing 81. With this arrangement, an electrical circuit is completed from the resistor track through brush 66, brush holder 68, brush 74 and plate 76 to terminal pin 78. The position of brush 66 can be varied on layer 48 by rotation of knob 70. A suitable stop (not shown) is formed in outer housing 73 and is preferably positioned in alignment with gap 83 between the end of the resistor track 42 and terminal 50 for engagement with radial arm 85 of the knob to limit the travel of brush 66 on the track, i.e. the brush is movable between a point in engagement with terminal 50 and the opposite end of resistor track 42.

It will be readily appreciated that the attenuator of FIG. 3 is the equivalent of the attenuator of FIG. 1, terminal pins 56 and 60 corresponding respectively to terminal pins 28 and 32 of FIG. 1 and brush 66 connected to terminal pin 78 being the equivalent of wiper contact 34 of FIG. 1.

In the foregoing description, it has been assumed that an outer resistive layer will be provided, i.e. layer 24 of FIG. 1 and layer 48 of FIG. 3, which layer has a specific resistivity substantially lower than that of the resistive body 12 of FIG. 1 and 44 of FIG. 3. However, it is believed that a practical working construction can be achieved, at least for some applications, with the elimination of the resistive layer and using only a single integral resistor bar representing both the portions of high and low specific resistivity provided that it is coated on one of its surfaces with a conductive layer such as layer 22 or 46 and that a terminal corresponding to terminal member 50, or layer 30 and terminal 32, is provided which engages only a portion of the integral resistor bar and is physically separated from the conductive layers 22 and 46.

Although this invention has been illustrated and described in connection with particular embodiments thereof, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

What I claim is:

1. A resistive attenuator comprising, in combination,

a support base of electrically insulating material,

a composite resistor bar in said base and comprising, a body portion of resistive material having oppositely facing surfaces and a layer of resistive mate- 'rial having an exposed surface and an oppositely facing surface in confronting electrical engagement with a surface of said body portion, said layer characterized by having a specific resistivity lower than that of said body portion,

first conductive terminal means on the other of said surfaces of said body portion and including a portion extending exteriorly of said base to provide a common input and output terminal for said composite resistor bar,

second conductive terminal means in said base and including a terminal portion extending exteriorly of said base, said second terminal means engaging said layer and electrically insulated from said first terminal means and said resistive body portion, said second terminal means providing an input terminal for said resistor bar and the electric circuit between said first and second terminal means being completed, through said layer and body portion,

a movable contact in engagement with and supported for movement on the exposed surface of said layer toward and away from said second terminal means,

and the thickness of said body portion to the thickness of said layer having a ratio of approximately 2:1 and said layer and first terminal means extending extensively over the oppositely facing surfaces of said body portion.

2. The resistive attenuator of claim 1 wherein said layer and first terminal means extend coextensively over the oppositely facing surfaces of said body portion.

3. A resistive attenuator comprising, in combination,

a body of resistive material having oppositely facing surfaces and relatively spaced end edge portions,

first terminal means having extensive electrical engagement on one of the surfaces of said body and providing a common input and output terminal for said resistive body,

a layer of resistive material having an exposed surface and an oppositely facing surface in confronting electrical engagement with the opposite surface of said body, said layer extending substantially coextensively on said opposite body surface and characterized by being relatively thin as compared to said body and having a specific resistivity substantially lower than that of said body,

second terminal means engaging an end edge of said resistive layer so that the electrical circuit from said first terminal means to said second terminal means is through said layer and said resistive body,

a contact member in engagement with the exposed surface of said layer, a

and means for supporting said contact member for movement on the exposed surface of said layer toward and away from said second terminal means, with an electrical input connected across said first and second terminal means and an output connected across said first terminal means and said contact member said body of resistive material and said layer of resistive material providing useable resistance values in the circuit.

4. The resistive attenuator of claim 3 wherein said body of resistive material is generally arcuate and said layer of resistive material is coextensive with said opposite body surface.

5. The resistive attenuator of claim 3 including a support base of electrical insulating material,

said body of resistive material being disposed in said support base,

said first terminal means extending through said base,

and said support base electrically insulating said sec- 6 and terminal means from said first terminal means 2,811,621 10/1957 Mucher et a1 338-174 X and said resistive body. 3,165,714 1/1965 Dreyfus 338--309 X References Cited by the Examiner FOREIGN PATENTS UNITED STATES PATENTS 5 634,073 8/1936 Germany. 2,680,177 6/1955 Rosenthal 338-138 X RICHARD WOOD, Primary Examiner- 2,704,316 3/1956 De Bell 338--174 R. F. STAUBLY, Assistant Examiner. 

3. A RESISTIVE ATTENUATOR COMPRISING, IN COMBINATION, A BODY OF RESISTIVE MATERIAL HAVING OPPOSITELY FACING SURFACES AND RELATIVELY SPACED END EDGE PORTIONS, FIRST TERMINAL MEANS HAVING EXTENSIVE ELECTRICAL ENGAGEMENT ON ONE OF THE SURFACES OF SAID BODY AND PROVIDING A COMMON INPUT AND OUTPUT TERMINAL FOR SAID RESISTIVE BODY, A LAYER OF RESISTIVE MATERIAL HAVING AN EXPOSED SURFACE AND AN OPPOSITELY FACING SURFACE IN CONFRONTING ELECTRICAL ENGAGEMENT WITH THE OPPOSITE SURFACE OF SAID BODY, SAID LAYER EXTENDING SUBSTANTIALLY COEXTENSIVELY ON SAID OPPOSITE BODY SURFACE AND CHARACTERIZED BY BEING RELATIVELY THIN AS COMPARED TO SAID BODY AND HAVING A SPECIFIC RESISTIVITY SUBSTANTIALLY LOWER THAN THAT OF SAID BODY, SECOND TERMINAL MEANS ENGAGING AN END EDGE OF SAID RESISTIVE LAYER SO THAT THE ELECTRICAL CIRCUIT FROM SAID FIRST TERMINAL MEANS TO SAID SECOND TERMINAL MEANS IS THROUGH SAID LAYER AND SAID RESISTIVE BODY, A CONTACT MEMBER IN ENGAGEMENT WITH THE EXPOSED SURFACE OF SAID LAYER, AND MEANS FOR SUPPORTING SAID CONTACT MEMBER FOR MOVEMENT ON THE EXPOSED SURFACE OF SAID LAYER TOWARD AND AWAY FROM SAID SECOND TERMINAL MEANS, WITH AN ELECTRICAL INPUT CONNECTED ACROSS SAID FIRST AND SECOND TERMINAL MEANS AND AN OUTPUT CONNECTED ACROSS SAID FIRST TERMINAL MEANS AND SAID CONTACT MEMBER SAID BODY OF RESISTIVE MATERIAL AND SAID LAYER OF RESISTIVE MATERIAL PROVIDING USEABLE RESISTANCE VALUES IN THE CIRCUIT. 